1. Field of the Invention
The present invention relates to improvements in or relating to machines for densifying vegetable matters, such as wood waste, waste from the agrifoodstuffs industry, and the like and for making combustible logs or billets.
2. History of the Related Art
All vegetable matter contains, inter alia, cellulose and lignin; the latter begins to plasticize, liquefy, at about 100.degree. C.; it is therefore of interest to produce, in the densifying machines, a temperature greater than 100.degree. C. in the mass of the vegetable matter.
In fact, the lignin being liquid, the particles of vegetable matter are coated therewith and, during cooling of the logs or billets, the lignin performs the role of natural binding agent on resolidifying.
After cooling, the logs or billets consequently have a natural protective film against atmospheric humidity, a greater mechanical strength, an excellent homogeneity and their shape remains stable during combustion.
Different types of machines of continuously densifying vegetable matter exist, incorporating; piston, rotating drum, screw, and those deriving from U.S. Pat. No. 2,224,212.
Vegetable waste frequently contains foreign bodies and in particular silica coming for example from the bark of trees, or earth contained in the agri-foodstuff waste. The foreign bodies wear certain members of the machines more rapidly.
Known densifying machines deriving from U.S. Pat. No. 2,224,212 comprise a compression head whose function is to create a high pressure and a considerable rise in temperature by friction on the front face and within the mass of vegetable matter located between the head and the extrusion die; the temperature in the mass of vegetable matter may attain and exceed 180.degree. C. The compression head is the wearing piece of the machine.
With a view to reducing the temperature of the compression head and to avoiding too rapid destruction thereof, it has been provided in the prior art machines to cool this head.